Compact electrostatic precipitator for droplet aerosol collection

ABSTRACT

An electrostatic precipitator has a high voltage electrode including multiple wire segments that are positioned within a surrounding electrically conductive porous media having a central axis and wherein the electrode assembly extends along the central axis. The electrode assembly has a plurality of wire lengths positioned to extend in a direction along the longitudinal axis of the porous media, and the wire segments being arranged to have a substantially longer total length than the length of extension along the longitudinal axis. An aerosol containing droplets is passed into the interior of the porous media, and across the electrode, which is charged with a high voltage. The porous media is at a substantially lower or different voltage from the high voltage electrodes. Flow of the aerosol containing particles charged by the electrode passes through the porous media to the outlet and the charged particles are precipitated by the porous media. Electrostatic shields are provided around high voltage insulators to reduce the likelihood of contamination of the insulators, which causes unsatisfactory current leakage.

BACKGROUND OF THE INVENTION

This invention relates to droplet aerosol collection by electrostaticprecipitation, and methods that improve efficiency for particlecollection. The improvements include one or more of the use of multiple,thin wire discharge electrodes; the use of a conductive porous medium asa collecting surface; the use of high voltage electrostatic shield toprevent particle deposition on the insulator for the components; and theuse of heated insulator to prevent vapor condensation and particledeposition by thermophoresis.

Electrostatic precipitation is one of the most widely used methods forremoving suspended particulate matter from a gas for gas cleaning or airpollution control. In comparison with other particulate collectingdevices, such as cyclones, wet scrubbers, filters, and the like, anelectrostatic precipitator has the advantage of low pressure drop, highcollection efficiency and requiring relatively small amounts ofelectrical power for its operation. The low pressure drop of theelectrostatic precipitator makes the device most advantageous with largevolumetric flow rates of the gas flow needing treatment. Electrostaticprecipitation have been used extensively for large scale industrialapplications, such as removing fly ash from power plants, controllingparticulate emission from smelters, steel and cement making and othersimilar industries, and general purpose air cleaning for buildingventilation. A typical electrostatic precipitator may operate at severalhundred cubic feet per minute of flow in small systems, to severalmillion cubic feet per minute for large industrial installations.

The first laboratory demonstration of electrostatic precipitation wasmade by Hohifeld in 1824, according to credible sources. The first U.S.patent on electrostatic precipitation was issued to Walker in 1886 asNo. 342,548. Numerous other electrostatic precipitator patents have beenissued over the years. Those considered the most significant includeU.S. Pat. No. 895,729 to Cottrell on the use of rectified alternatingcurrent for electrostatic precipitation, and the invention of the liquidfilm precipitator by Bums as shown in U.S. Pat. No. 1,298,088; the finewire electrode and two-stage precipitation system of Schmidt, U.S. Pat.No. 1,329,285; the low-ozone air-cleaning precipitator of Penney U.S.Pat. No. 2,000,654; and pulse energizing of precipitators disclosed inU.S. Pat. No. 2,509,548 to White, among others.

The fundamental design of the electrostatic precipitator has remainedrelatively unchanged over the years. In its simplest form for a singlestage precipitator, a high voltage electrode is placed in the center ofa grounded tube. A high DC voltage on the small diameter centerelectrode causes a corona discharge to develop between the electrode andthe interior surface of the tube. As the gas containing suspendedparticles flows between the electrode and the wall of the tube, theparticles are electrically charged by the corona ions. The chargedparticles are then precipitated electrostatically by the electric fieldonto the interior surface of the collecting tube.

One disadvantage of the electrostatic precipitator is its relativelylarge physical size. According to Deutsch (W. Deutsch, Ann. der Physik,Volume 68, p. 335, 1922), the basic equation governing the operation ofthe electrostatic precipitator is:

η=1−e^(−A w/Q)

The Deutsch equation relates the precipitator collection efficiency, η,to the collecting area of the precipitator, A, the volumetric flow rate,Q, through the precipitator, and the electrical migration velocity, w,of the particles. e is the constant, 2.718, the base of naturallogarithms. For a specific application, the collecting area of theprecipitator, A, is determined when the required volumetric gas flowrate, Q, is known. To reduce the overall physical size of theprecipitator, closely spaced precipitating plates can be used. However,there is a limit on this approach to reducing physical size. When theresulting physical size of the precipitator is still too large for theapplication, an electrostatic precipitator is then consideredunfeasible.

Several applications have developed in recent years where a significantreduction in the overall physical size of the electrostatic precipitatoris needed. One application is the removal of the suspended particulatematter from the blowby gas from a Diesel engine. In Diesel engines, thehigh temperature, high pressure combustion gas in the engine cylinderhas a tendency to leak past the piston rings into the crankcase. This isusually referred to as the blowby gas. This blowby gas containslubricating oil droplets from the lubricating oil films atomized by thehigh velocity blowby gas flowing from the high pressure cylinder intothe crankcase. It also contains Diesel exhaust particulates, whichresult from the incomplete combustion of the Diesel fuel in the enginecylinder. The amount of blowby gas is relatively small for new engines,but will increase over time as the engines age, and the piston rings nolonger provides a good seal. This blowby gas usually has a flow rate offew cubic feet per minute to perhaps as high as 20 cfm for engines ingood operating condition.

The Diesel blowby gas is currently being exhausted directly into theatmosphere. In order to protect the environment, there is a need toremove suspended oil droplets and Diesel exhaust particulates in theblowby gas so that the blowby gas can be returned to the fresh airintake side of the Diesel engine for further combustion. This “blowbygas recirculation system” is practical only when the suspendedparticulate matter is removed to avoid contaminating the components andequipment located on the air intake side of the Diesel engine. One suchcomponent is the turbo charger or compressor used to supercharge theDiesel engine to increase its power output and efficiency.

For application in the blowby gas recirculation system, theelectrostatic precipitator must be compact and reliable. It is alsodesirable that the operating voltage of the precipitator be relativelylow so that very a high supply voltage is not needed.

Another application for an electrostatic precipitator that is reduced insize from existing precipitators is for removing suspended oil andgrease particles in the exhaust gas from commercial kitchens, includingkitchens in fast-food, as well as conventional, restaurants.

A third application of an electrostatic precipitator of reduced size isto remove cutting fluid droplets from the machine shop environment.During machining of metal parts, a cutting fluid is usually directed atthe tool and the parts being machined to provide cooling as well aslubrication. Some of this cutting fluid is aerosolized to form smalldroplets by the higher speed rotary cutting tool. This cutting fluidaerosol presents a heath hazard to the workers and must be filtered toremove the suspended droplets. Conventional fibrous filters are notsuitable for this application, because the collected droplets tend toclog the filter and produce excessive pressure drop in a short time. Theinherent advantage of the small compact physical size and the inherentflame arresting properties of the precipitator of the present inventionmakes it particularly suited for these applications.

It should be noted the term “compact size” is used here in a relativesense to indicate that the size of the precipitator designed on thebasis of this invention is smaller or more compact in comparison withelectrostatic precipitators of a conventional design at the same flowrate and at the same efficiency level. By necessity, as a diesel blowbyparticle collector, the electrostatic precipitator must be sufficientlysmall to fit under the hood of a truck powered by a diesel engine. Theoverall volume of the collector must be no more than a few liters,preferably below two liters. On the other hand, an electrostaticprecipitator designed for kitchen exhaust applications will need to beconsiderably larger because of the high flow rate of the exhaust gas tobe treated. Such a collector can also be called compact even though thecollector is several cubic feet in total volume so long as the collectorof the conventional design is even larger, perhaps by as much as 50 or100%.

SUMMARY OF THE INVENTION

The present invention is an electrostatic precipitator that has improvedoperating efficiency while being smaller in physical size than existingdevices that handle similar flow rates. The present device uses multipleelectrical wire discharge electrodes which permit reducing the length ofthe precipitator. An electrically conductive porous medium is preferablyused as the collecting surface. A further aspect of the invention is anelectrostatic shield used to reduce or prevent particle deposition onthe insulators for high voltage components. A further aspect of theinvention is use of heated electrodes which prevent vapor condensationand also prevent particle deposition by thermophoresis.

All aspects of the invention cooperate to increase efficiency and reducephysical size for a given flow rate. These improvements have made itpossible to significantly reduce the overall physical size of theprecipitator. The small, compact physical size has in turn made itpractical to use electrostatic particle collection for the aboveapplications where small physical size is important. Treating dieselblowby exhaust to remove suspended oil droplets and particulate matterpermits the blowby exhaust gas to be discharged to the ambient withminimal amount of particulate air pollutant, or to be returned to theair intake side of the diesel engine for exhaust gas recirculation. Whenused to remove oil and grease particles contained in the exhaust ofcommercial kitchens the organic particulate matter will be removed.Another application is collecting droplet aerosols of cutting-fluid inmachine shops where sprayed liquids enter the atmosphere.

While the present invention was primarily developed for applicationssuch as those described above, the small compact size of the newprecipitator makes the device suitable for a variety of otherapplications, even in those cases where small physical dimensions arenot a primary requirement.

For the purpose of this disclosure, Aerosol is defined as smallparticles suspended in a gas. The particles can be a solid, a liquid, ora mixture of both. The particle size can range from approximately 0.001μm to 100 μm, with 0.01 μm to 20 μm being the size range of the greatestinterest. For the present application, most of the mass of aerosolparticles to be collected is concentrated in the latter size range.Droplet aerosol is defined as an aerosol in which the suspendedparticles are primarily in a droplet form and having a spherical shape.However, the liquid droplets need not be a pure liquid, and may containsuspended solid particles within each droplet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a compact electrostaticprecipitator made according to the present invention;

FIG. 2 is a sectional view taken on horizontal line 2—2 in FIG. 1;

FIG. 3A is a schematic sectional view of a modified form of theelectrode support and high voltage shield used with the precipitator ofFIG. 1;

FIG. 3B is a further modified form of a electrode support and highvoltage shield used with the precipitator of FIG. 1;

FIG. 4 is a transverse sectional view of a precipitator made accordingto the present invention but having a rectangular configuration;

FIG. 5 is a schematic representation of an ultrasonic generator used forintroducing aerosols into the electrostatic precipitator in the presentinvention;

FIG. 6 is a cross sectional view of a modified compact precipitatorusing a different style of electrode assembly from that shown in FIG. 1;

FIG. 7 is sectional view taken on line 7—7 in FIG. 6;

FIG. 8 is a sectional view of a still further modified form of aelectrostatic precipitator of the compact electrostatic precipitator ofthe present invention;

FIG. 9 is a sectional view taken on the line 9—9 in FIG. 8;

FIG. 10 is a schematic block diagram of a blowby gas recirculationsystem used in a diesel engine;

FIG. 10A is a modified recirculation system similar to that shown inFIG. 10;

FIG. 11 is a further modified block diagram of a blowby gasrecirculation system used in a diesel engine;

FIG. 12 is a block diagram similar to FIG. 11 with a controlled flowrestrictor on the outlet of the intercooler;

FIG. 13 is a cross sectional view of a modified support for theelectrode wire;

FIG. 14 is a vertical sectional view of a further modified compactelectrostatic precipitator;

FIG. 15 is a sectional view taken on line 15—15 in FIG. 14;

FIG. 16 is a cross sectional view of a modified support for theelectrode wire as it would be taken along the line 15—15 of FIG. 14;

FIG. 17 is a cross sectional view of a modified support for theelectrode wire as would be taken along the line 17—17 of FIG. 14; and

FIG. 18 is a flat layout of a cylindrical electrode support unrolled toa flat surface to reveal a modified pattern for the electrode wiresupported on the electrode surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic cross sectional view of an electrostaticprecipitator 10 made according to the present invention. A housing 12has a discharging electrode assembly 14 to produce the corona discharge.The high voltage DC power supply 16 applies a high voltage (severalthousand volts), to the electrode assembly 14 on a wire surrounded by aninsulator bushing 18. The bushing 18 is surrounded by a high voltageshield 20, made of suitable conducting material.

An electric heater 22 is in contact with the insulator bushing 18 tokeep the insulator bushing at a sufficiently high temperature to preventvapor condensation and particle deposition on the bushing 18.

Gas containing suspended droplets and other particulate matter from asource 23 is directed to flow through an inlet opening 24 of the housing12 and passes through a porous medium 26 in the inlet. The porous medium26 is a relatively inefficient droplet collector to keep out largecontaminants, so that most of the droplets in the aerosol are carried bythe gas into the electrostatic electrode region or chamber 28 above.

The input gas then flows around the electrode assembly 14 to expose thedroplet particles in the gas to the high electric field around theelectrode assembly. The discharge electrode assembly 14 includes acentral rigid support 30 for two support discs 32 and 34 on oppositeends of the central support. The upper disc 32 may be attached to theinsulator bushing 18 and thus support the discs 32 and 24 from thehousing 12. A plurality of holes 35 (see FIG. 2) are formed in each disc32 and 34 and a fine metal wire, 36 is strung between them. Thesectional view FIG. 2, through the compact electrostatic precipitatorelectrode 14 shows there are eight holes in each of the support discs. Afine metal wire 36 is threaded through the holes to form eight straight,parallel discharge electrodes 36. By way of example, if the distancebetween the two support discs is 8 inches, the fine wire electrode 36extending between them will each be 8 inches in length for a totaldischarge electrode length of 64 inches. More holes can be used in thesupport discs 32 and 34 to create more discharge electrodes, or fewerholes can be used if less length of the discharge electrodes is needed.With the above mentioned distance of 8 inches between the support disks,an electrode circle diameter of 3 inches, the diameter of the housing 12is approximately 5 inches, and its length, approximately 10 inches.Using the conventional design of a single discharge electrode in thecenter of a tube, the total length of the electrostatic precipitator ismore than 64 inches. The advantage of the present electrode design inreducing the size of the precipitator and making it compact over theconventional design thus becomes obvious.

The gas (aerosol) flows around the wires 36 and ions are produced in thecorona discharge. The ions collide with the droplets to cause thedroplets to be charged. The charged droplets are then carried by the gasflow through an electrically conducting, grounded porous medium 40 asthe gas flows to an outlet 42. The droplets are collected byelectrostatic precipitation onto the grounded collecting elements in themedium.

The clean gas then flows out of the annular space 41 between the porousmedium 40 and the outer housing 12 to the outlet 42. The collected oildroplets flow down the inside surface of the porous medium 40 as a thinfilm which is returned by gravity to an oil reservoir or sump 44.

As shown in FIG. 1, all parts of the system are grounded except for thehigh voltage electrode assembly and the high voltage shield 20.

Using a thin wire of a uniform diameter in the above electrodearrangement, and in other embodiments disclosed, it is important to keepthe distance between each wire segment and the adjacent collectorelectrode the same for all the wire segments on the support structure.By keeping the distance uniform and using the same high voltagepotential on all the wire segments, a uniform corona discharge can bemaintained. This will insure that all particles flowing through thedevice will be charged uniformly and to the same maximum possible extentto insure high collection efficiency for the device.

In designing an electrostatic precipitator using the above electrodeassembly, the spacing, S, between the wire segments must bear a properrelationship to the distance, D, between the wire segment and theadjacent collector electrode surface. (see FIG. 2). Too small a spacing,S, will cause the closely spaced wire segments to interfere with eachother, thereby reducing the maximum current that can be obtained fromeach wire. Too large a spacing will cause some empty spots on thecollector electrode surface to appear. Within these empty spots, thereare no corona current flow. Particles flowing over these empty spotswill not encounter corona ions and thus remain uncharged. Fromexperience, it has been found that the ratio, S/D, must be kept betweenthe limits of 0.1 and 10, preferably between 0.3 and 3, for theelectrode assembly to function properly and avoid degradation inperformance.

For application in a Diesel blowby gas recirculation system, the inlethousing 24 is connected to an opening in the crankcase, which isrepresented at 23, and the collected oil film is also returned directlyto the crankcase. The outlet 42 can be open to the atmosphere to allowthe cleaned blowby gas to be discharged to the atmosphere, or the outlet42 can be connected to the intake of the Diesel engine for exhaust gasrecirculation.

The total discharge electrode length is greatly increased from that ofthe conventional precipitator with a single discharge electrode in thecenter of a tube. The corona current that can be maintained between thedischarge electrode and the collecting tube is generally proportional tothe total electrode length. The approach described here makes itpossible to greatly increase the electrode length and hence the totalcorona current, thereby increasing the efficiency for both dropletparticle charging and precipitation of the charged droplets orparticles. A laboratory prototype device has demonstrated thepracticality of this approach. As many as sixteen discharge electrodeshave been used leading to approximately a factor of sixteen increase intotal corona current in laboratory prototypes.

Another purpose of the electrode design shown is to allow the dischargeelectrode to be circumferentially supported on a circle. A largediameter circle of the electrode length mounting will bring thedischarge electrodes (the wires) closer to the porous medium 40collecting surface, thereby reducing the voltage needed to maintain thecorona discharge between the electrode and the grounded porouscollecting surface. A lower operating voltage from existingprecipitators is desirable for the applications described above, toreduce the need for very high voltage insulation. When using a lowervoltage, the leakage current through the insulator bushing 18 can bereduced. Using a lower voltage also reduces the cost and complexity ofthe power supply 16, thus making the device more economical to produce.In the present device, voltages of between 5,000 to 10,000 volts aremost preferred, but voltages up to 20,000, volts DC can be used.

Using a circle of electrode lengths spaced from the center rod alsoforces the gas flowing radially outward toward the porous collectingsurface to be exposed to the very high electric field surrounding eachdischarge electrode. Generally, the electric field strength according toGauss's law tends to decrease with increasing distance from thedischarge electrode. The closely spaced wires forming the dischargeelectrodes forces the gas to pass through the high field region betweenthe electrodes and to be exposed to the high electric field around thewires. Each droplet or particle can thus be charged to a higher levelthan is possible with the conventional single length electrode design,thereby gaining a higher electrical charge and allowing droplets to bemore easily removed by electrostatic precipitation.

Although a porous collector electrode 40 is shown in FIG. 1 as thecollector electrode, the basic design of the discharge electrodeassembly 14 works well also when the collector electrode is made of asolid conducting material, in which case the housing 12 itself can bethe collector. The oil droplets will be collected on the interiorsurface of the housing walls. The collected oil droplets will then flowdown the walls and be returned to the oil sump or the crankcase of thediesel engine, eliminating the porous collector electrode will make thedevice less efficient, but the overall size, the complexity, and thecost of the device will also be reduced.

The high-voltage insulator bushing 18, if unprotected, will be exposedto the suspended droplets or particles in the gas, as well as anycondensable vapor which may be present. Over time, the accumulation ofdeposited and condensed material on the insulator will render itineffective. The insulator is heated by contact with the electricalheating element 22 to a high enough temperature to prevent vaporcondensation on the insulator bushing.

To prevent the precipitation of droplets or particles on the insulatorbushing surface, a conductive shroud or shield 20 surrounds theinsulator. This conductive shroud 20 is connected to the same highvoltage source as the discharge electrodes 36 so that a high electricfield is created in the region between the shroud and the nearbygrounded surfaces of the porous medium 40 or housing 12. The chargeddroplets or particles present in the gas will thus be precipitated ontothe grounded surfaces and not on the high voltage insulation bushing.

Design variations of conductive shroud 20 are shown in FIGS. 3A and 3B.By using a small gap spacing between the bottom plate of the shield orshroud and the nearby grounded surface, a high electric field can becreated in this gap space to also precipitate droplets or particles inthe gas.

In FIG. 3A, the modified high voltage shield as indicated at 50, and asshown has a base plate 50A, and the surrounding wall 50B that surroundsthe insulator bushing 18. The grounded housing 12 has a cap portion 52that comes up from a top wall 54 and defines an opening near the upperend of the insulator 18, as shown. The surrounding wall 50B is spacedfrom the wall over cap 52, and terminates short of the upper end wall ofthe cap. Thus there is a gap shown at 56 between the shield wall 50B andthe housing wall 52 around the insulator. The support shown at 56supports a top plate 32 of the electrode assembly. The central supportand the lower electrode plate 34 can be provided as before.

In FIG. 3B, the high voltage shield comprises a flat disc 60 that isfixed to the lower end of the insulator bushing 18, and the insulatorbushing 18 in this case is also surrounded by a sleeve or cap 62 of thehousing, which is grounded.

The top wall 64 of the housing is spaced from the plate 60, to form agap 66 between the housing wall 64, which is a top wall, and the plate60 which is a shielding disc. The support 68 can be used for supportinga top plate 32 of the electrode assembly as before.

Each of these forms of conductive shroud shows a gap between the highvoltage shield or shroud and a portion of the grounded housing. The gapis relatively narrow, and will provide for precipitation of chargedparticles that come near the high voltage shield, to the walls of thegrounded housing.

Creating a long pathway in the gap space as shown in FIGS. 3A and 3B,the charged droplets or particles in the gas can be efficiencyprecipitated in the regions surrounding the insulator bushing 18 toprovide improved protection of the high voltage insulator fromparticulate contamination.

In spite of the efficient high voltage insulator shield design of thisinvention, there is the possibility that some droplets or particles inthe gas may remain uncharged. These uncharged particles will be capableof penetrating through the gap space 56 or 66 between the shroud and thenearby grounded surface to deposit on the insulator. The precipitationof these uncharged particles on the insulator can be prevented byutilizing the phenomena of thermophoresis. Thermophoresis refers to themovement of aerosol particles in the direction of a decreasingtemperature gradient due to the radiometric force acting on theparticles. For effective thermophoretic motion of the particles toprevent particle precipitation on the insulator the insulator must heheld at a sufficiently high temperature. The insulator temperature mustbe 10° C. or more than the surrounding gas temperature. In contrast, toprevent vapor condensation, the insulator only needs to be held abovethe dew point of the condensable species in the gas. Usually at least afew degree C above the gas temperature would be sufficient

To be effective, the porous medium 40 must be made of a conductivematerial, usually metal. It can be made of a perforated metal, a porous,sintered metal, one or more layers of wire mesh material rolled into thedesired cylindrical shape, a pad of metal fiber or wires formed into acylinder, and similar configurations. As the gas flows into the porousmedium, particles are brought to close proximity to the surface of theconducting elements in the medium, thus allowing the charged particlesto be effectively deposited onto the surface of the conducting elementsof the porous medium. In comparison, in the conventional electrostaticprecipitator using solid collecting electrodes, such as a solid tubesurrounding the center electrodes, the charged particles must beprecipitated by electrical force through the fluid boundary layeradjacent to the inner surface of the surrounding tube.

Depending on the gas flow velocity, the relatively stagnant boundarylayers adjacent to the solid collecting surfaces may be a centimeter ormore in thickness. The particles must be precipitated through thiscentimeter thick stagnant gas layer to be deposited on the surface. Incomparison, using a porous collecting electrode, as shown here, the gasis forced to flow between the closely spaced conducting elements in theporous medium, thereby greatly reducing the distance the particles musttravel to reach the collecting surface. This will increase theefficiency of the precipitator and reduce the overall physical size ofthe device.

Not all electrically conducting porous material can be used with thecompact electrostatic precipitator described in this invention. In orderto handle the high gas flow rate per unit of collecting surface intendedfor this application, the porous material must not produce excessivepressure drop at the required high gas flow. In addition, the collectedoil drops must drained off easily by gravity and not be collected in theporous medium to clog the medium or produce excessive high pressuredrops. Depending on the structure of the porous medium, and the surfacetension and viscosity of the liquid droplets being collected, thedistance between the conducting elements of the porous medium must bekept above a critical limit. Too small a distance will allow thecollected droplets to form surface films bridging neighboring elementsand block the flow. For the usual liquid such as lubricating oils, themean distance between the conductive elements in the medium must belarger than about 5 microns, and preferably larger than 10 μm. The meandistance between the elements in a porous medium is also referred to asthe mean pore diameter which can be measured by a commercial poremeter.A mean pore diameter greater than 5 μm, preferably greater than 10 μm,is generally necessary for the medium to work successfully as the porouscollecting electrode of the droplet collecting precipitator describedherein.

There are a number of devices using a porous medium to collect chargedparticles. One such device is the electrically augmented bag filterdescribed by Penney in U.S. Pat. No. 3,910,779. In Penney's device, theparticles are charged in a corona charger. The charged particles arethen carried by the gas flow through a fabric medium and deposited onthe surface of the fabric. The particles to be deposited must be a drysolid material, so that the deposited particles on the fabric will forma porous cake. Since a cake will also form on the fabric in the absenceof an electrical charge, electrostatics charges are used by Penney tomodify the property of this cake namely to increase the pore size of thecake and reduce the pressure drop. The textile fabric used in a fabricfilter is usually not electrically conductive, so that it is notpossible to maintain a corona discharge directly between the coronaelectrode and the fabric. A separate corona charger is used upstream ofthe fabric filter to charge the particles for subsequent filtration bythe fabric.

Another device using a porous filter media is what is usually referredto as electrostatically enhanced fibrous filter such as that describedby Carr in U.S. Pat. No. 3,999,964. A conventional fibrous filter mediamade of glass, polymeric and other non-conducting fibers is sandwichedbetween two sets of electrical grids. A potential difference isestablished between the grids to create an electric field in the mediumto enhance the efficiency of the medium for particle collection byelectrostatic attraction. The device is most effective when theparticles are electrically charged. If the particles are not charged, acorona ionizer can be used upstream of the filter to charge theparticles to increase the efficiency of the filter for particlecollection.

A further version of the electrostatically enhanced fibrous filter isthat of Argo et al in U.S. Pat. No. 4,222,748. In Argo's device, acorona charger is used upstream to charge the particles. As the chargedparticles are collected in the fiber bed, which is made of anon-conductive material, charge will build up in the bed to raise itselectrical potential. To prevent the continuous buildup of charge in thebed, the bed is continuously irrigated by water to make the bedconductive. Particles collected in the bed are also carried away by theflowing water.

The electrostatic precipitator of the present invention is veryefficient and can be made into a small compact size. For manyapplications, such as diesel blowby filtration, the cylindrical geometrywith a circular cross section is the most convenient. However, it is notnecessary that the cross section shape be a circle to take advantage ofmany of the features of this invention. Rectangular, elliptical, andother cross sectional shapes can be easily adapted to the design of anelectrostatic precipitator described by the method described in thepresent invention.

FIG. 4 represents a transverse sectional view through a rectangularprecipitator. The electrode assembly 72 including a pair of spacedcorona wire supports 74 (only one is shown) would be made as before withthe two supports 74 spaced along a support rod 76 with wire 77 formingelectrodes extending between the supports. The wires 77 are shown in thecross over portions for threading through the holes. A conductive porousmedium collecting electrode 78, surrounds the high voltage electrodeassembly 72, and the porous medium, and the grounded outer housing 79have a generally rectangular cross-sectional shape.

In designing such a rectangular precipitator, it is important to keepthe individual corona wire lengths between the support 74 atapproximately the same distance from the porous collecting electrode 78.This will insure that the corona discharge between the high voltagecorona wire 76 and the collecting electrode 78 will be uniform at thesame applied voltage on the wires. As before, the lateral distancebetween the wire lengths and the porous collecting electrode 78 can bereduced to lower the required operating voltage of the precipitator.

Although the precipitator described in this invention is intended fordroplet aerosol collection, it can also be used to collect aerosolscontaining only dry solid particles. To prevent the build up of solidparticles in the porous collecting electrode which will cause pluggingof the pores, liquid droplets, usually water, can be added to theaerosol before it is introduced into the precipitator. FIG. 5 shows anultrasonic droplet generator 80 used in conjunction with anelectrostatic precipitator 82 for droplet addition. As aerosol flowsfrom source 84 through the ultrasonic generator 80, it picks up dropletsin the space 86 above an agitated liquid 88 produced by ultrasonicagitation using an ultrasonic transducer 89. The dry particulate matterwill be precipitated along with the added liquid droplets in theprecipitator 82 and be carried away by the liquid stream resulting fromthe collected droplets, thereby preventing the build up of dry solidmaterial on the collecting electrode in the precipitator. Other dropletgenerating devices, such as compressed air atomizer, bubblers, and thelike can also be used. The electrostatic precipitator can be made asshown in any of the forms disclosed

Because of the small droplet size and the large surface area of thedroplets produced by ultrasonic agitation or a compressed air atomizer,the combined wet electrostatic precipitator and droplet generatordescribed above will have excellent gas absorptive properties, and canbe used as a combined gas and particle scrubber. The combined gas andparticle scrubber will have a variety of applications in air pollutioncontrol. For instance, in the semiconductor industry, the exhaust gasfrom the vacuum pump downstream of a semiconductor process equipmentoften contains both toxic gases as well as fine particulate matter. Onesuch gas is fluorine, which is used at the end of a process cycle toclean the process chamber. Fluorine is very reactive to water and willbe efficiently scrubbed by water droplets in the combined dropletgenerator and wet electrostatic precipitator. Similarly, various acidicvapors such as hydrogen fluoride (HF) and hydrogen chloride (HCl) can beabsorbed by water droplets or by an aqueous solution of KOH and otherbasic solutions. By combining a droplet generator with appropriatechemical scrubbing solutions and the wet electrostatic precipitator, ahighly efficient combined gas and particle scrubber can be obtained.

FIGS. 6 and 7 show a compact two-stage electrostatic precipitator 98 inwhich an electrode assembly 100 including a short corona-dischargeelectrode 102 that is attached to a cylindrical precipitating electrode104, and both are held at the same high DC voltage from a voltage orpower source 106. The short corona-discharge electrode 102 has a pair ofspaced support discs 108 and 110 held together with a central support112. The discs support a fine wire 113 carrying a high voltage toproduce a corona-discharge. The cylindrical electrode 104 is a tubularcylinder with a conducting surface. This cylindrical electrode 104together with the surrounding porous metal media collector 114 form aprecipitating region in which the charged particles are precipitated.

In this two-stage design, the relatively short corona wire lengths 113Aforming electrodes produce a corona discharge to charge the droplets orparticles moving past the corona-discharge electrode 102. The shortlength of electrode 102 reduces the corona output from the wires, hencethe required current output from the power source 106 is reduced, inturn reducing its physical size, and cost. The design also makes itpossible to vary the radius of the circle of the corona wire lengths113A independently from that of the radius of the tubular cylinderelectrode 104. By changing these two radii, both the corona dischargeelectrode 102, which is an ionizer, and the precipitating cylinderelectrode 104 can be independently optimized, leading to improvedoverall operation of the system.

The discs 108 and 110 are held together with a central support 112. Thefine wire 113 is threaded between the discs 108 and 110, and carries thehigh voltage from the source 106. The high voltage again is carried bywire through an insulator bushing 118, which is surrounded by highvoltage carrying shield 120. An end plate 104A on tube 104 carries thevoltages to the tube 104. The tube 104 in turn is connected to the disc108 for powering the corona discharge electrode 102. The flow of gas isfrom an inlet 116 of housing 12 to an outlet 117, which discharges cleangas.

FIGS. 8 and 9 show a modified electrode design that can be used with thesingle-stage and the two-stage precipitators shown in FIGS. 1 and 6. Inthis case, a plurality of support rods 120 are attached to the supportdiscs 122 and 124 to form an assembly. A single corona fine wire 126 isspirally wound around the support rods 120 to extend from one disc tothe other, and this forms a plurality of segments of conductive wirecarrying current for supporting a corona discharge for charging particlein the droplet aerosol introduced through an inlet 128. The porous mediacollector 129 is shown in FIG. 1 with a coarse filter formed at a bottompanel 130, and selected porosity on a cylindrical electricallyconductive porous side wall media 132. The cylindrical side wall 132acts to precipitate charged droplets and particles as previously shown.The cylindrical wall 132 is grounded, as is the housing 12. An outlet134 from the housing discharges clean gas. The insulator bushing 18,heater 22 and voltage source are the same as shown before.

The compact electrostatic precipitator described herein can be used toremove suspended particles in the blowby gas from a diesel engine orother internal combustion engines. The blowby gas with the suspendedparticulate matter removed can be discharged directly into theatmosphere, or can be recirculated into the engine. FIGS. 10 and 11described below are both suitable for use for any electrostaticprecipitator, including that of the conventional design.

FIG. 10 shown one arrangement for blowby gas recirculation using anelectrostatic precipitator, preferably one made according to the presentinvention. The diesel engine 135 has a crankcase 136 and blowby gas fromthe engine crankcase 136 first flows along a passage through anelectrostatic precipitator 137 designed as show previously to removesuspended droplets or particles. The clean gas then flows into the inletsection of a T-connector 138 which has a orifice plate 138A in an outletsection. The gas flows through an orifice 140 in the plate 138A and intothe intake of a turbo charger 142. The side inlet section 136B of theT-connector 138 is open to atmosphere.

This T-connector constitutes a crankcase pressure regulating device whenan electrostatic precipitator is used to remove particles from theblowby gas for recirculation into the diesel intake. Its operation is asfollows. The T-connector 138 inlet 138B is open to the atmosphere, andthus the outlet of the precipitator 137 is also at atmospheric pressure.The crankcase pressure Pc relative to atmospheric pressure Pa is thusPc−Pa=ΔP, where ΔP is the pressure drop of the blowby gas through theprecipitator 137. This pressure drop is usually quite low, on the orderof a few inches of water or less. The crankcase pressure is thus limitedto a few inches of water above atmospheric. In an internal combustionengine, the crankcase pressure must not be allowed to vary by more thana few inches above or below atmospheric to prevent leakage of crankcaseoil to the outside, and other operational difficulties. This designmakes it possible to achieve crankcase pressure regulation with a simpleconnection and at low cost.

In a diesel engine using a turbo-charger or turbo-compressor to increasethe engine power output, as shown in FIG. 10, a filter 144 is used atthe air intake of the turbo-charger 142 to remove suspended particles inthe ambient air. The pressure drop through the filter 144 causes thepressure Pt at the turbo-charger intake to be below atmospheric. Thediameter of the orifice 140 in the outlet section of the T-connector 138is chosen such that the pressure drop across the orifice 140 (ΔP=Pa−Pt)is just sufficient to cause the gas flow through the orifice 140 to bethe same as the blowby gas flow Q1 during normal engine operation, andwhen the engine intake air filter 144 is new. When the intake filter 144becomes partially clogged, its pressure drop increases. This increasesthe gas flow through the orifice Q2. The difference, Q3=Q2−Q1 is made upby the air flow coming from the ambient through the side inlet section138B of the T-connector 138.

Alternatively, as shown in FIG. 10A, a modified orifice housing 139 canbe made as a straight through flow tube with no side inlet foratmospheric air. An atmospheric inlet 139A can be connected to anopening in the diesel engine crankcase 136.

In both the arrangements shown in FIGS. 10 and 10A, the blowby gaspassing through the electrostatic precipitator 137 is at a relativelyhigh temperature. It also contains oil vapor which is not removed byelectrostatic precipitation. This oil vapor will condense on the heattransfer surfaces of an intercooler 146 used at the outlet of theturbo-charger 142. Over time, the condensed oil will flood theintercooler 146 to cause a drop in the intercooler efficiency and thepower output of the diesel engine if not handled or removed.

To automatically remove this accumulated oil from the intercooler 146,an oil sump 148 is provided in the intercooler to allow the condensedoil to flow into the sump by gravity. The airflow from the intercooler134 is directed through a flow restriction 150, such as a nozzle or anorifice to create a pressure drop to remove the oil from the intercooler146 and be carried by the airflow into the engine intake. The oilcollected in the oil sump 148 can also be fed to the intake manifold 131of the engine 135, by the back pressure created by the flow restriction150.

FIG. 11 shows a second arrangement for recirculating the blowby gas intoa diesel engine 135. The crankcase 137 is connected to the electrostaticprecipitator 137 as before, but the T-connector 138 is removed and theflow from the precipitator 137 is directed to a filter intake plenum 154and allowed to pass through the filter 144 along with the intakeairflow. No crankcase pressure limiting arrangement is needed in thiscase. Since the precipitator outlet is always at atmospheric pressure,the crankcase pressure will thus be automatically limited to that neededto maintain the blowby gas flow through the precipitator 137.

When the hot blowby gas is directed this way into the filter intake 154,the oil vapor will be quickly cooled as it comes in contact with thecool collecting filter elements of the filter 144. The vapor will thuscondense and be collected in the filter housing. At the same time, allsubmicron size particles, which may not be completely removed by theelectrostatic precipitator, will also be subjected to the strongthermophoretic forces created by the temperature gradient in theboundary layer of the gas flow around the collecting elements of thefilter 144. this thermophoretic force can be effectively utilized toremove these submicron particles. Normal engine intake air filters aredesigned to collect particles larger than a few micron in diameter only.Small particles in the submicron size range are usually not collected.By utilizing the thermophoretic force, the fine particles in the blowbygas can also be collected, thus making the incoming air to theturbo-charger cleaner. With proper design, oil and fine particleaccumulation in the intercooler can be reduced to very low level.

FIG. 12 is similar to FIG. 11 and the parts that are identical areidentically numbered. In FIG. 12 a controllable flow restrictor 158 isconnected to the outlet of the intercooler 146. The flow restrictor hasa retractable vane or blade 158A that can be introduced into theinterior passage of the restrictor and which is controlled by a solenoid159. The solenoid 159 is connected to the vane or blade 158A and willextend the blade into the flow passage when a signal is received by thesolenoid. An oil level sensor 160 is provided on the oil sump 148, andwhen the oil level in the sump reaches a set level, the signal isprovided to energize the solenoid 159. The vane or blade 158A is movedinto the flow passage in flow restrictor 158 to restrict flow throughthe outlet line.

This action increases the back pressure in the oil sump and forces thecollected oil out a line 161 to the intake manifold 131 of the dieselengine. The solenoid controlled restrictor can be any desired form, sucha as a valve that closed partially, or an orifice that is introducedinto the flow passageway.

FIG. 13 is a sectional view of a modified version of typical electrodesupport 170. It can be molded from plastic and has an outer wall 172,with a plurality of projections or “prongs” shown at 174 which make theouter surface much like a serrated surface. A wire of suitable diameterindicated at 176 can be wound around the support 170 in a helicalfashion, much as shown in FIG. 8, with the points of the serrations orprojections supporting the wire 176 at closely spaced intervalsdepending on the spacing of the serrations to insure that the wire 176is maintained in a proper position relative to the collector electrode.

FIG. 14 is a vertical cross-sectional view of a modified form of acompact electrostatic precipitator 199. In this form of the invention, aconductive sleeve 200 forms a passage for fluid, with an inletconnection 202 for receiving an aerosol, and an outlet connection 203. Aflow passageway is defined by a plurality of openings 204 in a housingplate 206 that is supported on sleeve 206A, which is positioned at theupper end of the conductive sleeve 200, and is supported on a cap plate208 on a flange 210 formed on the end of the outer sleeve 200.

The support sleeve 206A has an open center, and an end insulator portion215 of a main electrode support 212 is mounted therein. The upper endinsulator portion 215 of the support 212 is supported on the cover 208in a suitable manner. The upper end insulator portion has a receptaclefor a heater assembly 216, which has heaters 218 mounted in a outerjacket 219 that is heat conducting and in contact with the insulatorportion 215. The outer jacket 219 can be made of copper, which is a verygood heat conductor, to distribute the heat uniformly to its outersurface and keep the insulator surface 213 hot and clean fromcontamination by vapor condensation and particle deposition. The topplate 220 is a heat insulator to reduce the heating power required tooperate the heater. The electrical power to operate the heater, usually12 or 24 volts, is carried by the electrical leads 221 passing throughthe top plate 220.

A power connection line 224 can be passed out through a central openingof a cap 222. As shown, a power supply 226 to provide the high voltagefor the discharge electrode can be potted in the cap 222 and theconnector line or rod 225 can be within the precipitator and does nothave to extend through the cap. The line 224 can be a relatively lowvoltage, for example, a 24-volt supply could be provided. The heaters218 also would be connected generally to a 24-volt supply.

The main support 212 includes a hollow center electrode support 214 thatcan be, for example, injection molded as a single piece with the mainsupport 212. The electrode support 214 has an interior passageway inwhich the high voltage connection rod or line electrode 225 extends, anda thin electrode wire 227 can extend for connection directly to theelectrode wire shown at 228 that, as shown, is helically wrapped aroundthe insulating support 214. The electrode wire 228 is shown larger thanactual size and is a thin wire as previously explained. The insulatingmaterial sleeve 214 may be attached to the main support 212 withsuitable screws threaded up into the support 212. The upper part of theinsulating support has a conducting sleeve 217, which can be made of ametal and connected to the same high voltage electrode wire 226. Theinsulating support 214 can have a cross section that is cylindrical, ifdesired, or as shown in FIG. 15, it could be rectangular with the outercollector electrode 200 also being rectangular with care being taken sothat at the corners there was a uniform spacing between the wire 228 andthe collector electrode.

The cross section can take any desired configuration, as long as thespacings are maintained for a corona discharge.

The aerosol flow would come in as shown by the arrow 234, and flow upand around the passageway 235 between the high-voltage electrode wire228 and the collector electrode 200. In this case, the collectorelectrode 200 is not a porous member, but is a solid member that caneither be stainless steel, for example, or could be a conductingplastic. As the flow passes through the space between the electrode wire228 and the collector 200, the particles are charged by the corona ionsproduced by the wire electrode 228. Some of these particles areprecipitated onto the collector 200 in this region. The remainingparticles are carried by the gas to the upper part of the assemblybetween the precipitator electrode 217 and the collector electrode 220,where they are precipitated onto the collector 220 by virtual of thehigh voltage on the electrode 217. The flow then goes up through theopenings 204, and out through the outlet 203 as shown. The main support212 and the electrode support 214 can be injection molded as a singlepiece, if desired, with conductors formed as slip-fit jackets, orwrapped wires. The heaters 218 are easily installed to maintain thetemperature of the insulator at a desired level.

The high temperature at the heaters keeps vapor that enters the spacebetween the sleeve 206A and the upper high voltage insulator portion 215from condensing on the surface 213 of the high voltage insulator portion215 in the region around the center portion 215. The heaters alsoprovide enough heat to tend to repel contaminant particles by thethermophoretic effect and prevent them from depositing on the surface213 of the high voltage insulator portion 215. The heaters 218 are inheat transfer, contacting relation to the insulator portion 215 and willmaintain the temperature of the surface 213 sufficiently high to preventcontaminant particles from building up on the surface of the insulatorportion. Preferably the temperature of the surface 213 of the insulatorportion 215 is 10° or more than the temperature of the gas in thevicinity of the insulating surface 213 inside the precipitator housing.

FIG. 16 is a transverse cross sectional view of a modified electrodesupport 250 taken on the same line as FIG. 15. FIG. 17 is a verticalcross sectional view of the modified electrode support 250. A wire 252forming the electrode is in contact with the surface 254 of theelectrode support 250 and in substantial conformity to it. The wire 252can be wound around the support 250 as shown, and made to adhere to thesurface 254 by using a suitable adhesive material. When adhesives areused the wire 252 can have various patterns.

One such pattern for the wire 252 is shown in FIG. 18 at 258. In FIG. 18a surface 262 of a support 260 has been unrolled to a flat surface toreview the wire pattern on the surface 262. The electrically conductivedischarge wire 264 is in contact with the support surface 262, which ismade of an electrically insulating material, such as a plastic orceramic. The wire electrode 264 is of a substantially uniform diameterand the distance between the wire segments and the adjacent collectorelectrode is substantially uniform along the length of the wire. With auniform distance between the wire 264 and the collector electrode, asubstantially uniform corona discharge can be maintained. All parts ofthe wire 267 can thus be utilized effectively to insure a high chargingefficiency in a small compact overall physical size for theelectrostatic droplet collector.

Another way of fabricating the thin wire discharge electrode is to use aflat, thin dielectric, generally plastic, having a thin film clad on theouter surface. The flat thin dielectric with a thin film on the outersurface can be similar to those used in fabricating flexible, electriccircuit boards. The electrode wire pattern on the surface can be etchedby photolithography. The thin film forming the pattern can then beapplied to the surface of the support structure by an adhesive. In sucha case, the wire will no longer have a circular cross section. Thelateral dimension of the etched electrode, however, must be sufficientlysmall to sustain a corona discharge at the applied high voltagematerial.

The compact electrostatic precipitators shown are intended primarily fordroplet aerosol collection. The high collection efficiency for thecompact size also make the precipitators suitable for collecting dryparticle aerosols. The collected dry particles will accumulate in theunit and the precipitators must be periodically shut down for cleaningand maintenance. This is usually acceptable for most applications.

The compact electrostatic precipitator described herein, though notnecessary for the application, is particularly attractive because of itscompact physical size and high collection efficiency.

The invention as described above is for the preferred embodiments.Others, who are skilled in the art, will see other possibilities andembodiments to accomplish the same objectives based on the principlesand approaches described in the present specifications.

What is claimed is:
 1. An electrode assembly comprising a dischargeelectrode, an adjacent collector electrode having a surface, and asupport structure having a longitudinal length and a transverse width,the discharge electrode comprising a plurality of spaced thin wiresegments supported on said structure with a total length substantiallylonger than the length and the width, said wire being substantiallyuniform in diameter, maintained at substantially the same distance fromthe adjacent collector electrode, and charged to a high voltagepotential relative to said collector, wherein the spacing “S” betweenwire segments and the distance “D” between each wire segment and thenearest point on the adjacent collector electrode surface are selectedso the ratio S/D is within the range of 0.3 to
 3. 2. The electrodeassembly of claim 1, wherein said wire segments comprise substantiallyparallel lengths of wires with adjacent wire segments spaced from eachother at a substantially uniform distance.
 3. The electrode assembly ofclaim 1, wherein the wire segments extend in a generally longitudinaldirection.
 4. The electrode assembly of claim 1, wherein said wiresegments extend in a generally transverse direction and substantiallyperpendicular to the longitudinal length.
 5. The electrode assembly ofclaim 1, wherein the wire segments extend in a generally spiral patternalong the longitudinal length.
 6. The electrode assembly of claim 1, andfurther including a precipitator electrode, said precipitator electrodehaving a conducting surface substantially parallel to said collectorelectrode and maintained at substantially the same high voltagepotential relative to said collector electrode as the dischargeelectrode.
 7. The electrode assembly of claim 1, wherein the collectorelectrode and the support structure for the discharge electrode are of agenerally cylindrical shape.
 8. The electrode assembly of claim 1,wherein the collector electrode and the support structure for thedischarge electrode are of a generally rectangular shape.
 9. Anelectrode assembly of claim 1, wherein the support structure for thedischarge electrode comprises a conducting material.
 10. An electrodeassembly of claim 1, wherein the support structure for the dischargeelectrode comprises an insulating material.
 11. An electrode assembly ofclaim 1, wherein said support structure for the discharge electrode hasan insulating surface.
 12. An electrode assembly of claim 11, whereinsaid wire is supported on the structure in substantial conformity to thesurface of the support.
 13. An electrode assembly of claim 1, whereinthe collector electrode comprises a porous conducting material.
 14. Theelectrode assembly of claim 1, wherein the discharge electrode includesan insulator bushing, and a wire supported in the insulator bushing andconnected to a power supply, the insulator bushing being supportedrelative to an outer housing.
 15. The electrode assembly of claim 1further comprising a precipitating electrode, and the dischargeelectrode and precipitating electrode being at the same high voltagepotential relative to the collector.
 16. The electrode assembly of claim15, wherein said precipitating electrode comprises a cylindrical tubehaving a conducting outer surface.
 17. An electrostatic precipitator incombination with a diesel engine having crankcase blowby, and aconnection to carry a flow of the blowby gas through the electrostaticprecipitator for removing suspended particulate matter including oildroplets from the blowby gas, said electrostatic precipitator comprisinga discharge electrode, an adjacent collector electrode, a housingcontaining the electrodes, an insulator, a conductor passing throughsaid insulator and carrying the high voltage to the discharge electrodeinside said housing, and a heater, said housing having an inlet andoutlet and carrying blowby gas from the inlet to the outlet, saidinsulator having an external surface in contact with blowby gas insidesaid housing, and said insulator being in contact with said heater tomaintain a surface temperature on the insulator at 10° or more above thetemperature of the blowby gas and maintained sufficiently high toprevent precipitation of oil droplets and contaminant buildup on saidinsulator surface from the blowby gas.
 18. The electrostaticprecipitator of claim 17, wherein said insulator being at leastpartially surrounded by a conductive shield, said shield being held at avoltage substantially the same as the voltage on the collectorelectrode.
 19. The combination of claim 17, wherein the housing outletis coupled to an inlet to the diesel engine.
 20. An electrostaticprecipitator comprising a housing having an inlet and an outlet; theinlet being coupled to a diesel engine crankcase to carry gases from thecrankcase through the electrostatic precipitator, a discharge electrodeassembly supported in said housing; a collector electrode comprising aporous conductive medium surrounding said discharge electrode, theporous conductive medium consisting of one of a sintered metal and ametal fiber pad each having pores therethrough with the pores havingmean pore diameters larger than 10 μm ; and an oil droplet containinggas flow from the diesel engine crankcase entering the inlet and flowingtoward the discharge electrode, through the surrounding collectorelectrode, and out the outlet of the housing, said discharge electrodebeing maintained at a high voltage potential relative to the collectorelectrode to provide a charge to particles, including oil droplets inthe gas flow that are collected on the collector electrode.
 21. Theelectrostatic precipitator of claim 20 and a precipitator electrodehaving a conductive surface substantially parallel to the collectorelectrode, said precipitator electrode being maintained at substantiallythe same high voltage potential relative to the collector electrode asthe discharge electrode.
 22. The electrostatic precipitator of claim 20and an insulator in the housing having a surface in contact with blowbygas, and a heater operated to maintain the surface temperature of theinsulator at least 10° above the temperature of the blowby gas.